1. Field of the Invention
The present invention relates to a burst-type transmission power control apparatus of a radio transmitter, more particularly, to a burst-type transmission power control apparatus suitable for use in a radio transmitter in a time division multiple access (TDMA) system.
2. Description of the Related Art
Technological developments in the field of mobile communication terminals in recent years have been remarkable, Particularly in terms of cellular telephone mobile communication systems which may typically be referred to as special mobile (GSM) systems designed for moving subscribers to utilize telephones. The transmission systems used by such mobile communication systems are shifting or have shifted from analog radio transmission systems to digital radio transmission systems in order to be compatible with networks based on digital exchange technologies, improve the sound quality, maintain the confidentiality of coded signals and raising the communication capacity.
In addition, low-and medium-altitude orbit mobile satellite communication systems have become commercially feasible in recent years as global digital mobile communication systems are connected to each other in a seamless fashion along with portable terminal technologies. As a result, the demand for such systems and such terminals is rapidly increasing.
In such mobile communication systems, the TDMA radio system is used for portable terminals for the purpose of simple communication in order to reduce the load of the hardware on the portable terminal.
Additionally, high speed automatic power control (APC) systems adapted to control the power output including a ramping waveform added thereto before and after a steady-state burst for controlling the transmission burst output level and preventing diffusion into adjacent channels of the transmission spectrum on a frequency base at the time of turning on/off the burst, has been used in order to maintain the quality of transmission/reception lines which can handle a certain level of traffic intensity and allow high density multiple access within the same communication cell.
A first prior art burst-type power control apparatus is of a closed loop type which detects a part of transmission power by a detector and feeds it back tot he gate of a high power amplifier formed by a GaAs field effect transistor (FET) or a Si bipolar transistor (see: FIG. 9 of JP-A-5-152977). This will be explained in detail later.
In the above-described first prior art burst-type transmission power control apparatus, however, since the dynamic range of the detector is narrow, it is difficult both to increase the speed of the rising and falling characteristics of a transmission burst signal, and to increase the power level of a transmission burst signal.
A second prior art burst-type transmission control apparatus further includes a variable attenuator in the closed loop of the first prior art burst-type transmission control apparatus (see: FIG. 10 of JP-A-5-152977). The attenuation degree of the variable attenuator is controlled so as to cause the maximum input level of the detector to be constant, thus broadening the dynamic range o the closed loop, so that the operation of the detector is stabilized on a reproducible basis. This also will be explained in detail later.
In the above-described second prior art burst-type transmission control apparatus, however, the control sensitivity of the high power amplifier is not improved, thus exhibiting a high damping coefficient and giving rise to an overshoot or undershoot during the rising burst period.
A third prior art burst-type transmission control apparatus further includes a variable power driver amplifier at a prestige of the high power amplifier of the second prior art burst-type transmission control apparatus, so as to suppress fluctuation of the control sensitivity of the high power amplifier depending on the output power level (see: FIG. 1 of JP-A-5-152977 and JP-A-10-172380). This also will be explained in detail later.
In the above-described third prior art burst-type transmission control apparatus, however, phase fluctuations occur in the high power amplifier because abrupt fluctuations in the amplitude envelope which occur inside the high power amplifier at and near the burst rising and falling time periods intersects the above phase fluctuation region.
In FIG. 1, which illustrates a first prior art burst-type transmission control apparatus (see: FIG. 9 of JP-A-5-152977), a modulated signal Sin generated from a modulated wave generator 1 which is, in this case, a voltage controlled oscillator (VCO) is transmitted to a high power amplifier 2. As a result, the high power amplifier 2 is driven by the modulated signal Sin to generate a transmission burst signal Sout via a directional coupler 3. The transmission burst signal Sout is radiated from an antenna 4.
The directional coupler 3 takes out a part of the transmission burst signal Sout, and a detector 5 formed by a diode detects the output signal of the directional coupler 3. A relative error amplifier 6 compares a reference voltage Vref as shown in FIG. 2A with the detection voltage Vdet Of the detector 5 as shown in FIG. 2B to generate an error voltage Verror in accordance with the difference between the reference voltage Vref and the detection voltage Vdet.
The high power amplifier 2 is constructed by a GaAs field effect transistor (FET) having a gate for receiving the error voltage Verror, a grounded source and a drain for receiving a power supply voltage Vp from a power supply battery 7.
Thus, the transmission burst signal Sout is fed back by a closed loop of the detector 5 and the relative error amplifier 6 to the high power amplifier 2, so that the transmission burst signal Sout as shown in FIG. 2C is brought close to the reference voltage Vref as shown in FIG. 2A. In other words, when Vref where the level of the transmission burst signal Sout is higher than a desired level, the relative error amplifier 6 decreases the error voltage Verror, thus decreasing the level of the transmission burst signal Sout. On the other hand, when Vdet less than Vref where the level of the transmission burst signal Sout is lower than the desired level, the relative error amplifier 6 increases the error voltage Verror) thus increasing the level of the transmission burst signal Sout.
The reference voltage Vref is generated from a control unit 8 which receives a control signal Scont from a base station or the like. The control unit 8 convolutes a rectangular envelope waveform at a steady-state time period defined by time t2 and time U, a rising ramping envelope wave form at a rising time period defined by time t1 and time t2, and a falling ramping envelope waveform at a falling time period defined by time U and time t4 on a time basis. The rising and falling ramping envelope waveforms are helpful in removing the spurious spectrum of the transmission burst signal Sout due to the switching of the GaAs FET of the high power amplifier 2.
Note that waveforms of the reference voltage Vref depending on the control signal Scont are stored in a read-only memory (ROM) or a random-access memory (RAM) of the control unit 8 in advance.
In the case of fixed envelope modulation such as Gaussion filtered minimum shift keying (GMSK) modulation which is a angular modulation intrinsically free from amplitude fluctuations unlike xcfx80/4 shift quadrature phase shift keying (QPSK) modulation for personal digital cellular (PDC), there is no need for relative error control to select a relatively large time constant for the loop amplifier including the relative error amplifier 6 for smoothing the detection voltage Vdet including the amplitude fluctuations after detecting the envelope of the transmission burst signal Sout, taking the averaged power into consideration. In other words, it is possible to select a relatively small time constant in advance and specify only design parameters for the purpose of performing feedback control, of the saturation power level and hence high speed operation of the automatic power control (APC) loop.
In the burst-type transmission power control apparatus of FIG. 1, however, since the dynamic range of the detector 5 is narrow, it is difficult both to increase the speed of the rising and falling characteristics of the transmission burst signal SO, and it is difficult to increase the power level of the transmission burst signal Sout.
Also, when a low transmission power level is selected for the high power amplifier 2, the detection sensitivity fluctuates depending on its temperature.
Further, when a low transmission power level is selected for the high power amplifier 2, the output gate control sensitivity of the high power amplifier 2 becomes high and fluctuates, with the result that low-power APC control cannot be carried out stably on a reproducible basis.
In FIG. 3, which illustrates a second prior art burst-type transmission control apparatus (see: FIG. 10 of JP-A-5-152977), a variable attenuator 9 is inserted between the directional coupler 3 and the detector 5 of FIG. 1. The variable attenuator 9 is controlled by a control voltage Va from the control unit 8, so as to make the maximum input level of the detector 5 constant. Therefore, if the control sensitivity of the detector 5 is held to a substantially constant level by arranging the variable attenuator 9 upstream relative to the detector 5, it is then possible to provide accurate transmission power control over a wide dynamic range from a low power output to a high power output simply by means of a closed loop formed by the high power amplifier 2, the directional coupler 3, the variable attenuator 9, the detector 5 and the relative error amplifier 6. Thus, the dynamic range of the closed loop is broadened so that the operation of the detector 5 is stabilized on a reproducible basis.
In the burst-type transmission control apparatus of FIG. 3, however, since the control sensitivity of the high power amplifier 2 is not improved, in particular, since the control sensitivity of the high power amplifier 2 is high when a low transmission power level is selected for the high power amplifier 2, a higher open loop gain and a broader loop band are obtained, thus exhibiting a high damping coefficient and giving rise to an overshoot or undershoot during the rising burst period.
In more detail, generally, as for the relationship between the error voltage Verror and the transmission burst signal Sout or the output voltage of the high power amplifier 2, the control sensitivity defined by the power increment per the error voltage increment of the high power amplifier 2 is in the form of a curve which is high when the latter is producing allow power output and falls to a saturation level at a point near the maximum power output.
Thus, the control sensitivity of the high power amplifier 2 changes to a large extent, depending on the transmission power level.
In particular, the control sensitivity-rises to make rapidly, causing closed loop to oscillate spuriously in the manner described above when the output power level is low.
Note that the above-mentioned oscillation of the closed loop, which occurs when a low output power level is selected can be eliminated by appropriately selecting certain other constants to narrow the loop band and adopting a large time base for the closed loop, in order to delay its response. However, using this technique for correcting the control sensitivity, the response of the closed loop is constantly delayed. In particular, since the control sensitivity of the high power amplifier 2 falls when the maximum power output is selected, the response of the closed loop is greatly delayed at that time to make it impossible to control the burst output including the ramping waveforms before and after the steady-state burst in a desired manner. As a result, it is no longer possible to expect the effect of APC on various environmental changes including fluctuations in the supply voltage, the input level and/or the power gain of the high power amplifier 2.
Thus, in the burst-type transmission control apparatus of FIG. 3, if there is a resistance/inductance/capacitance (RLC) constant in the closed loop, which can cause the phase to undergo an extreme turn within the loop band, no safe margin can be secured for the phase nor for the amplitude, and a spurious oscillation may occur because of the positive feedback generated at and near the frequency where a higher-order curve of the open loop transfer characteristics intersects the 0 gain.
In FIG. 5, which illustrates a third prior art burst-type transmission output control apparatus (see: FIG. 1 of JP-A-5-152977 and JP-A-10-172380), a variable power driver amplifier 10 capable of regulating its gain is connected between the modulated wave generator I and the high power amplifier 2 so as to suppress fluctuation of the control sensitivity of the high power amplifier 2, depending on the output power level.
In FIG. 5, in addition to the feedback control for controlling the output of the high power amplifier 2 by means of the error voltage V error from the relative error amplifier 6, feed forward control of controlling the output level of the variable power driver amplifier 10 is carried out in such a way that the high power amplifier 2 may show a substantially constant control sensitivity level relative to the desired transmission power level. In other words, in addition to the closed loop feedback control adapted to maintain the detection sensitivity of the closed loop constant relative to fluctuations in the transmission power output, open control, i.e., feed forward control, is provided on the output level of the variable power driver amplifier 10 in response to the transmission power level so as to cause the control sensitivity of the high voltage power amplifier 2 to be at a constant level. Therefore, it is now possible to control the power output stably and accurately during the steady-state burst time period, including the rising and falling ramping time periods, over a wide dynamic power output range.
For example, when the power output should be stabilized by means of APC feedback by reducing the transmission burst signal, Sout from the high power amplifier 2 by 10 dB from the selected maximum output level, the input level of the high power amplifier 2 is reduced by 12 dB to cause the control sensitivity to shift from the curve C1 to the curve C2 in FIG. 4. As a result, the feedback operation can be carried out without modifying the error voltage Verror and the control sensitivity obtained when the maximum power output is selected, so that the effect of APC remains stable and accurate regardless of whether a low power output or the maximum power output is selected.
Similarly, when the power output should be stabilized by means of APC feedback by reducing the transmission burst signal Sout from the high output power amplifier 2 by 24 dB from the selected maximum output level, the input level of the high power amplifier 2 is reduced by 30 dB to cause the control sensitivity to shift from the curve C1 to the curve C3 in FIG. 4.
Thus, the feedback operation can be carried out without modifying the control sensitivity, with the result that the effect of APC remains stable and accurate regardless of whether a low power output or the maximum power output is selected.
The optimum control voltage V6 is selected in advance for each output power level of the high power amplifier 2 and stored in the ROM or RAM in the control unit 8, so that it can be retrieved from the ROM or RAM by means of an output power setting command when the corresponding output power level is selected in order to ensure a substantially optimum drive input. The subsequent operation of controlling the output stably and accurately is carried out by means of the closed loop feedback system.
However, when the transmission power control apparatus of FIG. 5 is applied to a terrestrial mobile communication terminal for global system for mobile communication (GSM), personal communications network (PCN) or personal communications system (PCS) using a GMSK modulation or a medium altitude orbit mobile satellite communication terminal utilizing a Gaussian filtered minimum shift keying (GMSK) modulation system, there remains a problem of phase error of the transmission burst due to phase fluctuations of the phase of the high power amplifier 2, which can occur in the burst-rising and-falling time periods.
Terrestrial mobile communication terminals and medium-altitude orbiting mobile satellite communication terminals employ a fixed envelope modulation system which ensures a high efficiency operation in the saturation region of the high power amplifier 2 when a high output power level is selected. Therefore, the steady-state operation point of the high power amplifier 2 is normally set in the saturation region. Under this condition, generally, a phase modulation (PM) region exists at and near the output saturation point of the high power amplifier 2.
As described above, phase fluctuations occur in the high power amplifier 2 because abrupt fluctuations in the amplitude envelope which occur inside the high output control amplifier 2 at and near the burst rising and falling time periods intersects the above phase fluctuation region.
It is an object of the present invention to provide a burst-type transmission control apparatus capable of reducing phase fluctuations, i.e., phase errors.
According to the present invention, a burst-type transmission output control apparatus, includes an open loop formed by a variable power driver for amplifying a modulated wave signal, a saturation type high power amplifier, and a directional coupler. A closed loop is constructed by a variable attenuator connected to the directional coupler, a detector and a relative error amplifier connected to the, saturation type power amplifier. A voltage converter supplies a power supply voltage to the saturation type high power amplifier so that a saturation of the saturation type high power amplifier is raised only for a short period before and after a steady-state burst waveform, including rising and falling time periods.